Overhead Medium Voltage Circuit Grounding

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Sunny_92

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Electrical Engineer
I'm trying to determine if the 2014 NEC would allow a solidly grounded overhead medium voltage circuit (pole line w/ bare conductors) without a neutral or ground conductor.

Looking at 250.184, I don't believe it would be allowed. Opinions? Thanks in advance.
 
How would it be solidly grounded without a neutral or a ground?
 
petersonra said:
It could be grounded without running the grounded conductor. Not an uncommon thing at 480 V.
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Corner grounded yes. But it is grounded at the pole, the pole has a neutral/ground.
I have to be missing something here…
 
electrofelon said:
I think he is talking about a solidly grounded SYSTEM, and taking two ungrounded phases of that without the neutral.
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Yeah, I don’t think that’s allowed on MV.
There’s a section about how to ground when the grounded point isn’t supplied at the service point, but I don’t think that applies.
 
Hv&Lv said:
Yeah, I don’t think that’s allowed on MV.
There’s a section about how to ground when the grounded point isn’t supplied at the service point, but I don’t think that applies.
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Yeah, utilities can do it but no go with the NEC. If you have a multi grounded neutral system, I would think just keep it a multi-grounded system, unless for some reason you want or need the higher line to line voltage.
 
electrofelon said:
I think he is talking about a solidly grounded SYSTEM, and taking two ungrounded phases of that without the neutral.
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Correct. I'm talking about a 15kV solidly grounded system where the neutral is grounded at the transformer but isn't run out with the phase conductors.

I've seen this at least once on a customer owned system. The engineer designed the overhead circuit with only 3 phase conductors and omitted any neutral or ground wire. The reasoning was to save money since there were no line-to-neutral loads, but I always felt it wasn't NEC compliant.
 
Sunny_92 said:
Correct. I'm talking about a 15kV solidly grounded system where the neutral is grounded at the transformer but isn't run out with the phase conductors.

I've seen this at least once on a customer owned system. The engineer designed the overhead circuit with only 3 phase conductors and omitted any neutral or ground wire. The reasoning was to save money since there were no line-to-neutral loads, but I always felt it wasn't NEC compliant.
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I looked again, and 250.186(B) is strange:

B) Systems Without a Grounded Conductor at the Service
Point. Where an ac system is grounded at any point and is not
provided with a grounded conductor at the service point, a
supply-side bonding jumper shall be installed and routed with
the ungrounded conductors to each service disconnecting
means and shall be connected to each disconnecting means
equipment grounding conductor terminal or bus. The supply-
side bonding jumper shall be installed in accordance with
250.186(B)(1) through (B)(3).
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But I am not sure what the other end gets connected to 🤔

There is also 250.90 and exception and 250.110 exception 2:

250.190 Grounding of Equipment.
(A) Equipment Grounding. All non–current-carrying metal
parts of fixed, portable, and mobile equipment and associated
fences, housings, enclosures, and supporting structures shall be
grounded.

Exception: Where isolated from ground and located such that any
person in contact with ground cannot contact such metal parts when
the equipment is energized, the metal parts shall not be required to be
grounded.

250.110 Equipment Fastened in Place (Fixed) or Connected by
Permanent Wiring Methods. Exposed, normally non–current-
carrying metal parts of fixed equipment supplied by or enclos‐
ing conductors or components that are likely to become
energized shall be connected to an equipment grounding
conductor under any of the following conditions:
(1)..…........

Exception No. 1: If exempted by special permission, the metal frame of
electrically heated appliances that have the frame permanently and
effectively insulated from ground shall not be required to be grounded.
Exception No. 2: Distribution apparatus, such as transformer and
capacitor cases, mounted on wooden poles at a height exceeding 2.5 m
(8 ft) above ground or grade level shall not be required to be grounded.
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electrofelon said:
I looked again, and 250.186(B) is strange:



But I am not sure what the other end gets connected to 🤔

There is also 250.90 and exception and 250.110 exception 2:
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Let's assume we're looking at a medium-voltage feeder downstream of the service point, so we can ignore 250.186.

Let's also assume that the overhead medium-voltage feeder is feeding pad-mounted transformers, so 250.110 Exception 2 doesn't apply.

My conclusion is that you either need an equipment grounding conductor and a single-point grounded neutral system [250.184(B)], or you need a neutral conductor and a multigrounded neutral system [250.184(C)].
 
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